Removing deposits from diaphragms employed in manganese electrowinning



United States Patent REMOVING DEPOSITS FROM DIAPHRAGMS EM- PLUYED IN MANGANESE ELECTROWINNING No Drawing. Application July 19, 1952,

Serial No. 299,916

3 Claims. (Cl. li -137) This invention relates to the art of electrowinning manganese from manganese sulfate-ammonium sulfate electrolytes. In that art direct electric current is passed through a manganese sulfate-ammonium sulfate electrolyte from anodes to cathodes and metallic manganese is deposited on the cathodes.

As is known, it is necessary to separate the electrolyte intotwo portions, catholyte and anolyte, since alkaline conditions are necessary for the deposition of manganese in the catholyte as distinguished from the acid conditions in the anolyte. Thus a typical pH range for the catholyte is from about 8.0 to 8.6 and for the anolyte from about 0.9 to 1.2, the former containing ammonium hydroxide and the lattersulfuric acid. To accomplish the required separation diaphragms are employed. Cotton canvas or duck has commonly been employed as diaphragm' fabric. The maximum effective life of this material, e. g. 6-8 weeks, is so short that the cost of replacement is unduly high and the present invention is directed to improvements which make possible 21 diaphragm life of as much as two years with consequent very substantial savings.

The diaphragm assemblies differ somewhat, depending on the type of cell employed. In one type the cathodes are stainless steel sheets about 18" x 36" suspended in a diaphragm assembly or chamber having sides, ends, bottom and an open top. In the prior art this was essentially a cotton canvas or duck bag with a rectangular wood frame positioned therein to maintain the parts in spaced relation to the cathode with the sides lying in planes substantially parallel to the cathode. The anodes are bars or rods spaced from the diaphragm assemblies. In another type of cell the relation of anodes and cat11- odesto the diaphragm assemblies or chambers is reversed, that is, the anodes are positioned in the diaphragm chambers. In that type of cell the diaphragm assemblies or chambers are similar to those above mentioned except that they are open at the top and bottom,

thebottom opening into a horizontal chamber partly defined by a false bottom parallel to and in spaced relation to the bottom of the cell. The anolyte is contained within the vertical diaphragm chambers or assemblies and the remainder of the cell above the constitutes the catholyte chamber.

As is known, the electrowinning process is continuous, that is, the cell is continuously fed with purified catholyte and spent anolyte is continuously withdrawn and employed to extract manganese in the form of manganous sulfate from suitably conditioned ore and the extract after purification is fed back to'the cell as catholyte. As the process proceeds continuously insoluble deposits are built up in and on the anode side surfaces of the diaphragm fabric, that is, the surfaces of the diaphragm fabric facing an anode in normal cell operation, and this phenomenon has created an obstacle to providing suitable -diaphragms having a. greatly improved life in relation: to cotton canvas or duck. Indeed it is in the false bottom then case of-the 'mor'e' durable fabrics thatthe problem arises.

, a drain pan.

2,739,869 Patented Mar. 27, 1956 ice 2 Thus, Jacobs et al., in Metal Industry, December 8, 1944, page 359, say:

Wool, glass, Vinyon and micropore rubber backed with Vinyon were tested in the pilot plant. All of these materials are acid resistant but since they also must be discarded after 6 weeks because of the deposited calcium sulfate they have no advantage over canvas.

The limited teaching of the prior art in respect of the diaphragm fabric and. the effective life thereof is also shown by the following quotation from Electrolytic Manganese and its Alloys, by Reginald S. Dean, New York, The Ronald Press Company, 1952, page 54:

Diaphragms-Canvas diaphragms have proved the most efiective and economical. The diaphragms at Boulder City were IS-ouncc non-waterproofed canvas.

The voltage drop in the diaphragms increases with use;

due to the deposition'of calcium sulphate and reaches 0.2 volt in six weeks, at which time inch of calcium sulphate will have accumulated on the diaphragm. At

this point the diaphragm is discarded.

In accordance with the present invention diaphragm fabric made of polyacrylonitrile yarn is employed and by cleaning the said fabric at regular intervals by the method of the present invention, the life of the diaphragms is extended to as much as two years with consequent very substantial economic and other improvements. That diaphragm fabric has the following specific and preferred structure: It is a woven fabric composed of Warp strands and filling strands consisting essentially of polyacrylonitrile yarn having warp strands and 23 filling strands per linear inch, said strands being multiply filament 200 denier yarns, the warp strands being s-ply and the filling strands being ill-ply, the said fabric having a Weight of about 18.5 ounces per squareyard. Fabric having the 1 stated physical and chemical characteristics possesses not 1 only the physical characteristics necessary for successful functioning as diaphragm material in the cell but also the stability necessary to maintain rthose characteristics throughout repeated use periods inthe cell and throughout,

repeated cleaning treatments as herein described.

The prior art has failed:

1. To appreciate the importance of cleaning the diaphragms at regular intervals and extending the life thereof.

2. To discover that not only calcium sulfate but also manganese dioxide causes plugging of the diaphrag'tns' by the deposits previously mentioned.

3. To provide an adequate method of cleaning which salts, loose metal, etc. The soaking time depends upon the amount of precipitated salts to be dissolved and is usually from 4 to 24 hours.

The second step is a treatment to remove calcium sulfate deposits from those surfaces of the diaphragm fabric which in cell operation face an anode. If the diaphragm assembly or chamber is one of the'type within which a cathode is positioned during cell operation, the said assembly or chamber bein open at the top and closed at the bottom, the said calcium sulfate insoluble deposits will form on the outside or anode side'surfacc's of the diaphragm fabric, since it is these surfaces which in cell operation face an anode. assembly or chamber is placed in an'upright'- position over It is filledwifthwatenland ismain'tained in In that case the diaphragm 2,739,869 I p I the filled condition by the delivery of water thereto. solvent action of the water which passes through the fabric destroys the bond which holds, the insoluble calcium sulfate deposits on the outside or opposite (the anode side) surfaces of the fabric, i. e. the surfaces which face an anode in normal cell operation, and those deposts either drop off or may easily be scraped from those surfaces. In this second step the time of treatment also varies from about 4 to 24 hours. If the diaphragm assembly or chamber is of the type within which an anode is positioned during cell operation, that is, open at the bottom as Well as the top, then the insoluble calcium sulfate deposits are on the inside (anode side) of the diaphragm fabric, i. e. the fabric surfaces facing an anode in normal cell operation, and the procedure of the second step of the cleaning method is modified as follows: The diaphragm assembly after removing from the cell is mounted in a tank which may be a dummy cell, that is, a tank similar to the electrolytic cell except for the absence of anodes, cathodes and electrolyte, water being used instead of the catholyte. A flow of water into and out of the said tank or dummy cell is preferably maintained because of the low solubility of calcium sulfate in water. In either case the principle of the second step of the cleaning method for removing difficulty soluble calcium sulfate deposits is the same, that is, a head of fresh water is maintained on the clean surfaces of the diaphragm fabric, that is, the surfaces facing a cathode in cell operation, and the combined solvent and pressure effects of the water which passes through the diaphragm fabric, loosen the calcium sulfate deposits so that they may be readily removed.

The third step is employed to dissolve manganese dioxide impregnated in the pores of the diaphragm fabric which the water does not remove in the second step. For that purpose the diaphragm assemblies or chambers are soaked in an aqueous MnOz solvent, preferably a solution of sulfur dioxide containing from about 0.2 to 5 gs. per liter of sulfur dioxide and, preferably, from about 0.5 to 5% by weight of sulfuric acid. The use of sulfuric acid in addition to sulfur dioxide in the solution of the third step is not mandatory but is preferred because the solvent action of the sulfur dioxide is facilitated by the presence of sulfuric acid. The solution may also contain from 5 to 20% of ammonium sulfate.

Following the third step the diaphragm fabric is preferably rinsed or washed with water and the diaphragm assembly is returned to the cell.

The frequency of the above described cleaning method The t as much as two years, contra to the belief and teachings of the prior art.

While the invention has been described .in terms of application to diaphragms made from polyacrylonitrile yarn, the cleaning method and its periodic use in obtain ing a long effective life of the diaphragm are also applicable to diaphragms made from synthetic polymeric yarns when the diaphragm fabrics have physical structure and chemical properties (including resistance to attack by the.

electrolyte) substantially the same and equivalent to the diaphragm fabric specifically described herein.

While the above 3-step method is preferred, the said steps may be varied as follows: In the first step instead of sulfuric acid, an aqueous solution containing from 5 to 20% byweight of hydrochloric or nitric acid can be used.

In the second step, instead of water, a solution containing about 5 to 20% by weight of nitric, sulfuric or hydrochloric acid may be used, or a solution containing 10 to 30% by weight of ammonium sulfate.

In the third step, instead of sulfur dioxide, or sulfur dioxide plus sulfuric acid or sulfur dioxide plus sulfuric acid and ammonium sulfate, other aqueous solvents for manganese dioxide may be used, e. g. a solution containing 5 to 10% by weight of ferrous sulfate and 5 to110%' by weight of sulfuric acid or a solution containing 5 to 10% by weight of hydrogen peroxide and 5 to 10% ,by' weight of sulfuric acid or a solution containing 5 to 10% by weight of oxalic acid and 5 to 10% by weight of sulfuric acid, or a solution containing 5 to 20% by weight of hydrochloric acid. All of those reagents have a reducing effect on MnOz which facilitates solution thereof.

Furthermore, all three steps may be combined into one step in a manner illustrated by the following method: The diaphragm assembly is removed from the cell, supported in an upright position and there is maintained on, the cleansurfaces of the diaphragm fabric, i. e. the'surf: faces facing a cathode in cell operation, a'hydtau'lic head of an aqueous solution containing one of the following; manganese dioxide solvents which also functions to ssen. the calcium sulfate deposits, for a period of 4 to 24 hours' until the calcium sulfate deposits on the opposite (anode) surfaces of the diaphragm fabric, i. e. the surfaces which face an anode in normal cell operation, are loosened and" until the manganese dioxide impregnated in the pores of said diaphragm fabric are dissolved.

1. An aqueous solution containing from 0.2 to 5 gs] per liter of S02, to which may be added from .5 to 5% by weight of sulfuric acid and 5 to 20% by weight'of am monium sulfate.

2. An aqueous solution containing from 5 to 10% by weight of ferrous sulfate and 5 to 10% by weight'of, H2304.

3. An aqueous solution containing from 5 to 10% by; weight of H102 and 5 to 10% by weight of H2804. Q

4. An aqueous solution containing 5 to 20% by weight of HCl.

The temperatures at which the herein described treat ments are carried out are not critical. A range 'of temperatures from 25 C. to 40 C. is preferred although temperatures below 25 C. and above 40 C. maybe employed, e. g. from about 10 C. to C.

While it is preferred to remove the diaphragm assent-1 bly from the cell for cleaning saidassembly, it is po's sible but not as convenient, to carryout the 'cleanin'g" methods within the cell. For that purpose it is neces- Q sary to short circuit the cell, drain the electrolytes, i.e. catholyte and anolyte therefrom, and to use the cell es.- sentially as a'tank or container for holdingor applying the cleaning reagents.

What is claimed is:

1. In the art of electrowinning manganeseusing electrolytic cell containing a manganese sulfate-airirfidf. nium sulfate electrolyte and a diaphragm assembly comf prising fabric made from polyacrylonitrile yarn supported{ on a framework toseparate the electrolyte into catholyte I and anolyte portions, and in which deposits offmanganese dioxide and calcium sulfate form on and in the interstices of said fabric during normal use of 'thedia phragm assembly, the method of cleaning said diaphragm, which comprises first removing the diaphragm assembly from the cell, immersing said assembly in a dilute sulil'f furic acid solution containing from about. 2 to: 10% by weight of sulfuric acid, for a period of about 4 to 24: hours; second, removing said assembly from said acid solution, supporting it in an upright position and maintaining the surfaces of the diaphragm fabric which fac'e a cathode in cell operation in contact with water uridrl. 1 a greater hydraulic head than acts on the OPPOsitesurfaces of said fabric until the calciurnsulfate -deposits formed on said opposite surfaces are loosened; th ir d ,.imi met-sing said diaphragm assemblyin an aqueoussolution 1.; of'sulfur dioxide containing from about 0.2 to 5 gs.

pas a per liter of sulfur dioxide and continuing said immersion for a period of about 4 to 24 hours to dissolve manganese dioxide impregnated in the pores of the diaphragm fabric, and, finally, washing said fabric with water.

2. In the art of electrowinning manganese using an electrolytic cell containing a manganous sulfate-ammo nium sulfate electrolyte and a diaphragm assembly comprising fabric made from polyacrylonitrile yarn supported on a framework to separate the electrolyte into catholyte and anolyte portions, and in which deposits of manganese dioxide and calcium sulfate form on and in the interstices of said fabric during normal use of the diaphragm assembly, the method of cleaning said diaphragm fabric to restore the effectiveness of said diaphragm fabric for repeated use in the cell, which comprises, first removing the diaphragm assembly from the cell, immersing said assembly in a dilute acid solution containing from about 2 to 10% by weight of an acid of the group consisting of sulfuric acid, nitric acid, and hydrochloric acid for a period of about 4 to 24 hours; second, removing said assembly from said acid solution, supporting it in an upright position and maintaining the surfaces of the diaphragm fabric which face a cathode in normal cell operation in contact with a calcium sulfate dissolving reagent of the group consisting of water, an aqueous acid solution containing 5 to by weight of sulfuric acid, an aqueous acid solution containing about 5 to 10% by weight of hydrochloric acid, and an aqueous solution containing about 5 to 10% by weight of nitric acid, said aqueous reagent being maintained under a greater hydraulic head than acts on the opposite surfaces of said fabric, until the calcium sulfate deposits on said opposite surfaces are loosened, and removing said deposits; third, immersing said diaphragm assembly in an aqueous manganese dioxide solvent of the group consisting of an aqueous solution containing from about 0.2 to 5 gs. per liter of sulfur dioxide, an aqueous solution containing from 5 to 10% by weight of ferrous sulfate and 5 to 10% sulfuric acid, an aqueous solution containing from 5 to 10% by weight of hydrogen peroxide and 5 to 10% by weight of sulfuric acid, an aqueous solution containing from 5 to 10% by weight of oxalic acid and 5 to 10% by weight of sulfuric acid, and an aqueous solution containing 5 to 20% by weight of hydrochloric acid, and continuing said immersion for a period of about 4 to 24 hours to dissolve manganese dioxide impregnated in the pores of said diaphragm fabric.

3. In the art of electrowinning manganese using an electrolytic cell containing a manganous sulfate-ammonium sulfate electrolyte and a diaphragm assembly comprising fabric made from polyacrylonitrile yarn supported on a framework to separate the electrolyte into catholyte and anolyte, and in which deposits of manganese dioxide and calcium sulfate form on and in the interstices of said fabric during normal use of the diaphragm assembly, the method of cleaning said diaphragm fabric to restore the effectiveness of said diaphragm fabric for repeated use in the cell, which comprises removing said diaphragm assembly from the cell, supporting it in an upright position and maintaining the surfaces of the diaphragm fabric facing a cathode in normal cell operation in contact with a manganese dioxide dissolving reagent of the group consisting of an aqueous solution containing from 0.2 to 5 gs. per liter of sulfur dioxide, an aqueous solution containing from 5 to 10% by Weight of ferrous sulfate and from 5 to 10% by weight of sulfuric acid, an aqueous solution containing from 5 to 10% by weight of hydrogen peroxide and 5 to 10% by weight sulfuric acid, and an aqueous solution containing 5 to 20% by weight of hydrochloric acid, until the calcium sulfate deposits on the opposite surfaces of the diaphragm fabric are loosened and until the manganese dioxide impregnated in the pores of said diaphragm fabric is dissolved.

References Cited in the file of this patent UNITED STATES PATENTS 442,335 Roberts Dec. 9, 1890 1,120,629 Salisbury Dec. 8, 1914 FOREIGN PATENTS 460,564 Canada Oct. 25, 1949 OTHER REFERENCES Jacobs et al.: Metal Industry, December 8, 1944, p. 359. 

1. IN THE ART OF ELECTROWINNING MANGANESE USING AN ELECTROLYTIC CELL CONTAINING A MANGANESE SULFATE-AMMONIUM SULFATE ELECTROLYTE AND A DIAPHRAGM ASSEMBLY COMPRISING FABRIC MADE FROM POLYACRYLONITRILE YARN SUPPORTED ON A FRAMEWORK TO SEPARATE THE ELECTROLYTE INTO CATHOLYTE AND ANOLYTE PORTIONS, AND IN WHICH DEPOSITS OF MANGANESE DIOXIDE AND CALCIUM SULFATE FORM ON AND IN THE INTERSTICES OF SAID FABRIC DURING NORMAL USE OF THE DIAPHRAGM ASSEMBLY, THE METHOD OF CLEANING SAID DIAPHRAGM WHICH COMPRISES FIRST REMOVING THE DIAPHRAGM ASSEMBLY FROM THE CELL, IMMERSING SAID ASSEMBLY IN A DILUTE SULFURIC ACID SOLUTION CONTAINING FROM ABOUT 2 TO 10% BY WEIGHT OF SULFURIC ACID, FOR A PERIOD OF ABOUT 4 TO 24 HOURS; SECOND REMOVING SAID ASSEMBLY FROM SIAD ACID SOLUTION, SUPPORTING IT IN AN UPRIGHT POSITION AND MAINTAINING THE SURFACES OF THE DIAPHRAGM FABRIC WHICH FACE A CATHODE IN CELL OPERATION IN CONTACT WITH WATER UNDER A GREATER HYDRAULIC HEAD THAN ACTS ON THE OPPOSITE SURFACES OF SAID FABRIC UNTIL THE CALCIUM SULFATE DEPOSITS FORMED ON SAID OPPOSITE SURFACES ARE LOOSENED; THIRD, IMMERSING SAID DIAPHRAGM ASSEMBLY IN AN AQUEOUS SOLUTION OF SULFUR DIOXIDE AND CONTAINING FROM ABOUT 0.2 TO 5 GS. PER LITER OF SULFUR DIOXIDE AND CONTINUING SAID IMMERSION FOR A PERIOD OF ABOUT 4 TO 24 HOURS TO DISSOLVE MANGANESE DIOXIDE IMPREGNATED IN THE PORES OF THE DIAPHRAGM FABRIC, AND, FINALLY, WASHING SAID FABRIC WITH WATER. 